1. Field of the Invention
The present invention relates to a discharge electrode, a discharge lamp using the discharge electrode and a method for manufacturing the discharge electrode, and more particularly to a discharge electrode serving as a hot cathode, a discharge lamp using the discharge electrode and a method for manufacturing the discharge electrode.
2. Description of the Related Art
A hot cathode (discharge electrode), used for discharge lamps such as fluorescent lamps, emit electrons from its surface in an atmosphere of a discharge gas by being thermally heated under application of negative potential to its surface. The hot cathode widely utilizes a filament implemented by a thin refractory metal wire, formed into a coil configuration, which is heated by electric energy. Furthermore, thermionic emission is generally promoted as the work function of cathode material thereof is decreased, and thus a variety of metals or materials called emitter materials such as a barium (Ba)-based materials have been formed on the surface of the filament, by a coating method, an impregnation method, or the like, in order to reduce the work function of the filament material surface.
For example, in a fluorescent lamp, which is the most widely and generally used discharge lamp, the flow of electric current in the hot cathode involves the dissipation of energy, heating the whole system of the hot cathode, and the thermionic emission is initiated from the surface of the hot cathode. In earlier technology, the hot cathode was fabricated by coating tungsten filament with a barium-based emitter material. Earlier hot cathodes, or earlier discharge electrodes make it possible to emit electrons via a small drop of the cathode voltage, which supports the high luminous efficiency of earlier fluorescent lamps, whereas earlier fluorescent lamps are associated with the problem of short operation life. Moreover, to satisfy the requirements for high integration of devices and needs for miniaturization, the development of a high-performance hot cathode operating at an even lower temperature and with lower heat dissipation is required to meet the requirements thereof.
Recently, in Japanese Patent Application laid-open No. H10-698688 (hereinafter called “the first document”), a discharge lamp installing a specific hot cathode (discharge electrode) has been proposed, the specific hot cathode has a layer of particulate diamonds on the surface of the hot cathode material. Namely, particulate diamonds having an average particle diameter of 0.2 μm or less are coated on the surface of the hot cathode material in the first document.
Further, in Japanese Patent Application laid-open No. 2000-106130 (hereinafter called “The second document”), another discharge electrode for integrating into a low-pressure discharge lamp has been proposed. In the second document, fine diamond particles having a particle diameter of from 0.01 μm to 10 μm, preferably from 0.1 μm to 1 μm, are deposited on or impregnated into the surface of a tungsten coil. The diamond-deposited or -impregnated tungsten coil was integrated into the low-pressure discharge lamp as the discharge electrode. The objective of the second document was to suppress the deterioration of thermionic emission characteristics of the discharge electrode and to achieve long operation life of the low-pressure discharge lamp.
The techniques disclosed in the first and second documents, however, are not sufficient in efficient improvement because the applied power is mostly dissipated at the tungsten coil.